A flow tube is lined with ETFE fluoropolymer, to resist corrosive effects of fluids being carried in the tube. The tube surface is preferably first coated with a metal having a greater affinity for the ETFE fluoropolymer than carbon steel or stainless steel, prior to applying the polymeric coating.
An ethylenetetrafluoroethylene copolymer (an ETFE fluoropolymer) is made by Dupont Company is sold under the trademark TEFZEL and made and sold under various names by other companies. The polymer is highly chemically resistant and mechanical abrasion resistant up to at least 300xc2x0 F. This resistance to both chemicals and mechanical impact makes it an excellent lining material for applications requiring protection of metals from chemicals or mechanical wear.
TEFZEL, that is, ETFE fluoropolymer, exhibits good room temperature bond strength to many types of metals, including 304 stainless steel, Hastalloy, carbon steel and other metals. This bond exhibits a tendency to weaken with increasing temperature and exposure to water. A combination of both high temperature and exposure to water tends to rupture the bond of the ETFE fluoropolymer to the metal rather quickly. The bond loss is caused by water permeating the ETFE fluoropolymer and attacking the bond between the ETFE and the metal wall.
The present invention relates to providing an ETFE fluoropolymer coating solidly bonded to metal, useful in many types of pipes and tubes, including flow meter tubes. The tubes are usually made with carbon steel or stainless steel. Direct application of polymers to such tubes does not provide a bond that is sufficiently resistant to rupture to work satisfactorily. Bonding a thin layer of another metal to the base tube material, which exhibits a strong bond to ETFE fluoropolymer sold by Dupont Company under the trademark TEFZEL and made and sold by various other companies. The coating provides a durable, damage and rupture resistant layer in the tube. In particular, the process used for coating the interior of a flow tube includes adding a thin deposited aluminum layer on the inner surface of a stainless steel tube, and then coating the tube inner surface by rotationally molding or molding by melting the ETFE fluoropolymer layer on the inner surface for resistance to chemicals and mechanical damage.
In one form of the invention, capacitor plates and sometimes combined with shield plates, driven to the same electrical potential as the electrodes, are supported in the ETFE liner in a suitable manner for use in a magnetic flowmeter and are connected to circuitry for providing an indication of flow rate of a liquid through the tube.
It is important to have a surface bond that retains the ETFE fluoropolymer layer in place, even upon quite wide temperature differentials. The coefficient of expansion of ETFE, for example, is approximately ten times that of most metals, and so that the metal to ETFE fluoropolymer bond is stressed upon cooling to room temperature after the ETFE has been heated and molded in place, and when cooled after a flow pipe lined with the ETFE has been used in high temperature environments, particularly when water is present. The liner will thus be prevented from forming a gap that water, which permeates the ETFE, will form in and collapse the liner upon heating and generating a steam pressure between the liner and tube wall. By applying a well bonded coating of metal that has affinity for, or that bonds well with, the ETFE fluoropolymer, such as aluminum, a long life bond is created so that the inherent properties of the polymer, such as resistance to abrasion and chemical attack are provided for a flow meter.